1. Field of the Invention
The present invention relates in general to glass touch sensing circuits, and more particularly to a glass touch sensing circuit which is capable of accurately detecting a sense signal resulting from a user's touch under no influence of variations in temperature.
2. Description of the Prior Art
Glass touch sensing circuits are generally adapted to input a command generated when the user touches a specific area on a display screen. These sensing circuits are used for the input of commands in a variety of household electric appliances and electronic products such as high-class microwave ovens, notebook computers, display screens, televisions, etc.
FIG. 1 is a circuit diagram showing the construction of a conventional glass touch sensing circuit. As shown in this drawing, the conventional sensing circuit comprises a touch sensor 5 for outputting a sense signal in response to a user's touch, and a resistor R1 and capacitor C1 interacting to convert the sense signal from the touch sensor 5 into a switching signal to a transistor Q1, which will hereinafter be described in detail.
The resistor R1 has its one end connected to an output terminal of the touch sensor 5 and its other end connected to a connection point A. Commonly connected to the connection point A are one end of the capacitor C1, one end of a resistor R4 and a base terminal of the transistor Q1. The other end of the capacitor C1, the other end of the resistor R4 and an emitter terminal of the transistor Q1 are connected in common to a first output terminal OUT1 of a microprocessor 15, which will hereinafter be described in detail.
The transistor Q1 is adapted to perform a switching operation in response to the switching signal converted by the resistor R1 and capacitor C1, so as to generate a low signal having a falling edge depth differently determined depending on a switching period of time. The conventional glass touch sensing circuit further comprises a resistor R3 and capacitor C2 interacting to remove a noise component from the low signal from the transistor Q1 and apply the resultant low signal as a clock signal to a D flip-flop 10, which will hereinafter be described in detail.
A resistor R2 has its one end connected to terminal 1 which inputs a supply voltage of 5V end connected in common to one end of the the other end of which is connected to the and a collector terminal of the transistor Q1. The low signal from the transistor Q1 is noise-removed by the resistor R3 and capacitor C2 and then applied as a clock signal to a clock terminal {overscore (CK)} of the D flip-flop 10.
The D flip-flop 10 is adapted to provide its output signal in response to the 5V supply voltage inputted by the input terminal 1, and the microprocessor 15 is adapted to provide its output signal for the control of the operation of the D flip-flop 10. The microprocessor 15 is further adapted to monitor the output signal from the D flip-flop 10 and recognize the user's contact with the touch sensor 5 as a result of the monitoring.
The D flip-flop 10 has an input terminal D, preset terminal {overscore (PR)} and voltage terminal VCC connected in common to the input terminal 1 which inputs the 5V supply voltage. The D flip-flop 10 further has an output terminal Q connected to an input terminal IN1 of the microprocessor 15. A second output terminal OUT2 of the microprocessor 15 is connected to a clear terminal {overscore (CLR)} of the D flip-flop 10, which also has a ground terminal GND connected to a ground voltage source. The first output terminal OUT1 of the microprocessor 15 is connected commonly to the emitter terminal of the transistor Q1, resistor R4 and capacitor C1, as stated above.
Next, a description will be given of the operation of the conventional glass touch sensing circuit with the above-entioned construction.
FIG. 2 is a timing diagram of output signals from the respective components in the conventional glass touch sensing circuit of FIG. 1.
The 5V supply voltage inputted by the input terminal 1 is always applied to the input terminal D of the D flip-flop 10. Under this condition, the microprocessor 15 provides an output signal OUT1 as shown in FIG. 2 at its first output terminal OUT1 at intervals of predetermined time to sense the user's touch with the touch sensor 5.
The output signal OUT1 from the microprocessor 15 is applied to the emitter terminal of the transistor Q1, thereby causing a potential difference to be generated between the emitter terminal and collector terminal of the transistor Q1 when the output signal OUT1 is low in level.
If the user touches the touch sensor 5 under the above condition, then a voltage being charged and discharged by the resistor R1 and capacitor C1 becomes higher in level due to an electrostatic capacity of the human body, thereby causing a turning-on period of time of the transistor Q1 to become longer than that when the user does not come into contact with the touch sensor 5.
In other words, because the transistor Q1 is switched in response to the voltage being charged and discharged by the resistor R1 and capacitor C1, it remains ON for a predetermined period of time even though the user does not come into contact with the touch sensor 5. However, in the case where the user touches the touch sensor 5, the amount of charge stored in the user is added to the voltage being charged and discharged by the resistor R1 and capacitor C1, resulting in an increase in the level of the charged and discharged voltage.
As a result, the voltage being charged and discharged by the resistor R1 and capacitor C1 when the user comes into contact with the touch sensor 5 is different in level from that when the user does not do so, thereby causing turning-on periods of the transistor Q1 in both cases to be different from each other.
Upon being turned on, the transistor Q1 generates a clock signal {overscore (CK)}, which is low in level as shown in FIG. 2, owing to a potential difference generated between its collector terminal and emitter terminal. At this time, the low signal generated by the transistor Q1 has a falling edge depth determined depending on a turning-on period of the transistor Q1. In other words, the low signal generated by the transistor Q1 while the user comes into contact with the touch sensor 5 has a falling edge depth greater than that while the user does not do so.
The low signal generated by the transistor Q1 is noise-removed by the resistor R3 and capacitor C2 and then applied to the clock terminal {overscore (CK)} of the D flip-flop 10.
The D flip-flop 10 outputs a high signal at its output terminal Q synchronously with the low signal, or low voltage, received at its clock terminal. It should be noted that, in a normal operation state, the D flip-flop 10 is enabled in response to a low signal applied under the condition of the user's contact with the touch sensor 5 and not enabled in response to a low signal applied under the condition of the user's noncontact with the touch sensor 5.
Then, the output signal from the D flip-flop 10 is applied to the input terminal IN1 of the microprocessor 15. Such a signal from the D flip-flop 10 is indicated by “INPUT SIGNAL IN1” in FIG. 2. Upon receiving the output signal from the D flip-flop 10, the microprocessor 15 recognizes the user's contact with the touch sensor 5 and then provides an output signal OUT2 at its second output terminal OUT2, which is low in level as shown in FIG. 2. The low signal from the microprocessor 15 is applied to the clear terminal {overscore (CLR)} of the D flip-flop 10 to clear the D flip-flop 10.
In other words, in the conventional glass touch sensing circuit, the D flip-flop 10 is enabled in response to a low signal applied to its clock terminal {overscore (CK)} when the user touches the touch sensor 5. Upon being enabled, the D flip-flop 10 outputs a high signal to the input terminal IN1 of the microprocessor 15, thereby causing the microprocessor 15 to recognize the user's contact with the touch sensor 5 and output a clear signal to the D flip-flop 10 so as to initialize it.
However, the above-described conventional glass touch sensing circuit has the following disadvantages.
The clock signal to the D flip-flop has no reference value which is accurately defined to enable the operation of the D flip-flop. This clock signal is defined only by the output signal from the transistor Q1 which has a falling edge depth varying according to whether the user touches the touch sensor 5. As a result, there is a concern that the D flip-flop could be different in enabling point according to individual specifications or manufacturers thereof, thereby causing each key in the touch sensor to be different in sensitivity and performance.
The conventional glass touch sensing circuit has a further disadvantage in that it varies in sensitivity with temperature. Namely, the transistor Q1, which is switched in response to the sense signal from the touch sensor 5, is so sensitive to variations in temperature that it remains ON for a lengthier period of time at a higher temperature and for a briefer period of time at a lower temperature, respectively. The output signal from the transistor Q1 varies with such a temperature variation, resulting in a variation in the low signal being applied to the clock terminal {overscore (CK)} of the D flip-flop 10. Consequently, the touch sensor becomes different in sensitivity according to variations in temperature.